1. Field of the Invention
The present invention relates to a power supply switch circuit with current leakage protection and, more particularly, to a circuit used in an electronic device and capable of preventing the erroneous actions caused by a circuit leakage current in high temperature environments.
2. Description of Related Art
As shown in FIG. 1, a prior art power supply switch circuit is primarily formed by connecting two transistors Q1, Q2 and other electronic components. When power is to be supplied, a trigger signal ST is used to control the transistor Q1 to be on so that a voltage source VCC can provide power to a load 90 via the transistor Q1. Meanwhile, the supplied power can also be sent to the base of the transistor Q2 to make the transistor Q2 on. After the transistor Q2 is on, the base of the transistor Q1 will be connected to a reference terminal Gnd to keep the transistor Q1 on. When the power is to be cut off, a controller 91 will send a cut-off signal to the transistor Q2 to make the transistor Q2 off. The transistor Q1 will then be off in succession so that the voltage source VCC can no longer provide power to the load 90.
The leakage current of a transistor depends on the working temperature. The magnitude of the leakage current is proportional to temperature. Please refer to FIG. 1 again. According to the transistor's characteristics, when the circuit stops providing power, the transistor Q2 will have a leakage current ICEO. When the temperature rises, the leakage current ICEO increases, and the base current IB1 flowing through the transistor Q1 increases therewith. Because the collector current IC is β times the base current IB according to the transistor's characteristics, the collector current IC1 of the transistor Q1 will increase with the base current IB1 of the transistor Q1. Similarly, the base current IB2 of the transistor Q2 increases with the collector current IC1 of the transistor Q1. With the increase of the base current IB2 of the transistor Q2, the leakage current ICEO increases. In this vicious circle, the power supply switch circuit will generate erroneous actions. That is, when the power supply switch circuit is originally in the off state (i.e., stops providing power), the transistor Q1 will be on due to the leakage current ICEO to drive the power supply switch circuit into the on state (i.e., starts providing power).